Conventionally, cellulose rayon is spun by a viscose process. As shown in FIG. 6, sodium hydroxide (NaOH) is added to pulp or raw cellulose material to form an alkali cellulose. After compressing and curing, carbon bisulfide (CS.sub.2) is added to xanthate the alkali cellulose to a xanthonic cellulose salt, then a diluted aqueous sodium hydroxide solution is added to dissolve the xanthonic cellulose salt to form a cellulose viscose. Then, the viscose is aged, filtered and induced into a spin-bath containing diluted sulfuric acid, sodium sulfide, zinc sulfide, etc. At the same time, a regenerated cellulose is formed by neutralization and regeneration with the components contained in the spin-bath, namely neutralization and regeneration with the diluted sulfuric acid, sodium sulfide, zinc sulfide, etc. After post-treatment, such as washing, bleaching and drying, a cellulose rayon is obtained. However, the necessary steps for conducting this conventional process are too complicated and time-uneconomical because the cellulose viscose can only be neutralized and regenerated in a spin bath instead of in the air. Additionally, hydrogen sulfide (H.sub.2 S), carbon bisulfide (CS.sub.2) and sodium sulfide (Na.sub.2 S) produced during the neutralization and regeneration and the metal zinc existing in the waste water will contaminate the environment. Furthermore, the resistance of the spin bath will decrease the velocity of spinning because the neutralization and regeneration of the cellulose viscose can only be done in the spin bath, which will lead to deterioration of the physical properties of the spun fiber. Therefore, the conventional viscose process is seldom applied recently for producing cellulose rayon.
Recently, in order to overcome the drawbacks of the conventional viscose process, a direct solvent-dissolving process has been developed. As shown in FIG. 7, raw cellulose material is dissolved in a solvent to form a cellulose solution. After regenerating, spinning, washing, and drying, a spun fiber is produced. The solvent which is suitable for the direct solvent-dissolving process is N-methyl morpholine oxide. Although this process has the advantages that the solvent can be recovered and re-used, higher speed spinning is possible, lesser containment is produced during neutralization and regeneration, and the physical properties of the spun fiber are much better, etc. the direct solvent-dissolving process still cannot be widely industrially applied because of the following reasons:
1. The raw cellulose material which can be used in the direct solvent-dissolving process is restricted to dissolving pulp which is used in the conventional viscose process and a further activation treatment of pulp is normally required. The source of this type of raw pulp is restricted and expensive, thus the direct solvent-dissolving process is not applicable for mass production; PA1 2. The viscosity of cellulose solution is as high as 12000-30000 poises which will induce the extending difficulty of spun fiber at high speed spinning because of the fluid resistance; PA1 3. The amount of raw cellulose material which can be dissolved in the solvent is relatively low, only in a range of 8-15%. Thus, the cost of applying this process is high because too much solvent is consumed; PA1 4. The decaying of the polymerization degree (DP) of cellulose is very high, which results from the operation conditions of high temperature and long dissolving time required for dissolving the raw cellulose material in the solvent. Thus, the tensile strength of the produced spun fiber decreases; PA1 5. The cost of the solvent is very high and the recovery yield of the solvent is low. The solvent decomposes easily at high temperature, thus the recovery yield of the solvent is only about 94%. PA1 a. the solvent mixture consists of 3 solvents of 40-70 wt % of N-methyl morpholine oxide (NMMO), 20-50 wt % of N-methylol caprolactam (NMC) and 5-35 wt % of tetra methyl ammonium chloride (TMAC); PA1 b. the raw cellulose material is dissolved in said solvent mixture by assistance of a thin film evaporator at 80.degree. C. and 50-150 torr; and PA1 c. the anti-polymerization degrading agent is 1000 ppm to 1% of stearyl-3(3',5'-di-tert-4-hydroxyphenyl)propionate based on the weight of the raw cellulose material. PA1 a. The operation temperature is kept below 90.degree. C. The present solvent mixture possesses high dissolving ability, thus it can dissolve more than 15% of cellulose at this temperature; PA1 b. The kneading is carried out under a low temperature. Additionally, a thin film evaporator is used to enhance the dissolving ability of raw cellulose material. PA1 c. An anti-polymerization degrading agent is added to reduce DP degradation.
Therefore, although the direct solvent-dissolving process is more preferable than the conventional viscose process, the above drawbacks still hinder its wide application in industry.